Ammonium, nitrate, and proton fluxes along the maize root

Ion-selective microelectrodes were used to measure NH₄⁺, NO₃^– and H⁺ fluxes along the primary root of maize seedlings. Plants were exposed to nutrient solutions containing NH₄⁺, NO₃^– or both ions. Nitrogen fluxes along the root varied substantially among the different treatments. Net NH₄⁺ and NO₃^– uptake and H⁺ extrusion were low at the very apex of the root and generally increased in the more basal regions. In the absence of nitrogen or in the presence of NO₃^– alone, net H⁺ uptake (and root surface alkalinization) occurred at the root tip (0–1 mm), whereas net H⁺ extrusion occurred in all other regions. In the presence of NH₄⁺ alone, a dramatic increase in net H⁺ extrusion was detected in all regions except for the region 6–11 mm from the apex. In contrast, when NO₃^– alone was supplied, net H⁺ extrusion was depressed at all locations except for the tip (0–1 mm). When both NH₄⁺ and NO₃^– were supplied, NO₃^– uptake was suppressed at all locations while net H⁺ extrusion was increased relative to NO₃^– alone. The capacities to absorb NH₄⁺ and NO₃^– at the tip were similar, as indicated by flux rates when NH₄⁺ or NO₃^– were supplied as sole sources, but when supplied together, net NO₃^– uptake was half that of net NH₄⁺ uptake, indicating that NH₄⁺ may satisfy the nitrogen requirements of the poorly vascularized apical tissue in the most energy-efficient way. The high spatial resolution of the measurements enabled us to establish that acidification in the root expansion zone is maintained regardless of nitrogen source.     

A comparative study of fluxes and compartmentation of nitrate and ammonium in early-successional tree species

¹³NO₃^– and ¹³NH₄⁺ compartmental analyses were carried out in seedling roots of trembling aspen (Populus tremuloides Michx.), lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and interior Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) at 0·1 and 1·5 mol m^–3 external NO₃^– or NH₄⁺ concentrations ([NO₃^–]_o or [NH₄⁺]_o, respectively). At the lower [NO₃^–]_o, the capacities and efficiencies of acquisition and accumulation of NO₃^–, based upon NO₃^– fluxes and cytoplasmic NO₃^– concentrations ([NO₃^–]_c), were in the order aspen >> Douglas-fir > pine. At 1·5 mol m^–3[NO₃^–]_o, the NO₃^– influx increased 18-fold in pine, four-fold in Douglas-fir and approximately 1·4-fold in aspen; in fact, at 1·5 mol m^–3[NO₃^–]_o, the NO₃^– influx in pine was higher than in aspen. However, at high [NO₃^–]_o, efflux also increased in the two conifers to a much greater extent than in aspen. In aspen, at both [NO₃^–]_o, approximately 30% of the ¹³N absorbed was translocated to the shoot during 57 min of ¹³N loading and elution, compared with less than 10% in the conifers. At 0·1 mol m^–3[NH₄⁺]_o, influx and net flux were in the order: aspen > pine > Douglas-fir but the differences were much less than in NO₃^– fluxes. At 1·5 mol m^–3[NH₄⁺]_o, NH₄⁺ influx, efflux and [NH₄⁺]_c greatly increased in aspen and Douglas-fir and, to a much lesser extent, in pine. In aspen, 29 and 12% of the ¹³N absorbed was translocated to the shoot at 0·1 and 1·5 mol m^–3[NH₄⁺]_o, respectively, compared with 5 to 7% in the conifers at either [NH₄⁺]_o. These patterns of nitrogen (N) uptake, particularly in the case of NO₃^–, and the observed concentration responses of NO₃^– uptake, reflect the availability of N in the ecological niches, to which these species are adapted.     

A comparison of NH4+ and NO3– net fluxes along roots of rice and maize

Net fluxes of NH₄⁺ and NO₃^– along adventitious roots of rice ( Oryza sativa L.) and the primary seminal root of maize ( Zea mays L.) were investigated under nonperturbing conditions using ion-selective microelectrodes. The roots of rice contained a layer of sclerenchymatous fibres on the external side of the cortex, whereas this structure was absent in maize. Net uptake of NH₄⁺ was faster than that of NO₃^– at 1 mm behind the apex of both rice and maize roots when these ions were supplied together, each at 0·1 mol m^–3. In rice, NH₄⁺ net uptake declined in the more basal regions, whereas NO₃^– net uptake increased to a maximum at 21 mm behind the apex and then it also declined. Similar patterns of net uptake were observed when NH₄⁺ or NO₃^– was the sole nitrogen source, although the rates of NO₃^– net uptake were faster in the absence of NH₄⁺. In contrast to rice, rates of NH₄⁺ and NO₃^– net uptake in the more basal regions of maize roots were similar to those near the root apex. Hence, the layer of sclerenchymatous fibres may have limited ion absorption in the older regions of rice roots.     

Nitrogen cycling and proton fluxes in an acid forest soil

Ironhill, near Liphook, UK, was the site of a forest fumigation experiment. Nitrogen cycling within the humoferric podzol soil was a component of the study into the impacts of sulphur dioxide and ozone on coniferous trees. Variation in total soil N and N mineralisation was too great to determine impacts from the fumigant gases. Differences in the nitrogen mineralisation potential of the soils were unrelated to the initial levels of mineral or total N, or to pH. Mineralisation potential was affected by temperature and a Q₁₀ of approximately 3 was demonstrated. Mineralisation potential was reduced in very dry soils, but the wetting of these dry soils did not result in enhanced mineralisation, relative to fresh samples of equivalent moisture content. Nitrification potential was detected in this forest soil of pH 3 (in 0.01 m CaCl₂).The soil N data and those from the analysis of N within vegetation were used to prepare N budgets for the second and third seasons' growth of a mixed conifer forest; by the third year, N appeared to limit tree growth.The relative magnitude of proton fluxes from plant growth, nitrification and atmospheric inputs was estimated. Acidity generated from the balance of cations and anions in plant uptake, and soil N transformations was estimated to be comparable to that from `acid rain'. This comparison was based on only parts of the N cycle because they may occur remotely, in time or space, from other transformations of N. The comparison is valid, therefore, at the scale of individual trees or small-scale experimental plots, but at forest scale, wet and dry deposition were predicted to be the more significant for ecosystem acidification.     

Ammonium and nitrate uptake by Eucalyptus nitens: effects of pH and temperature

Ammonium and nitrate uptake by roots of Eucalyptus nitens was characterised with respect to pH and temperature. Uptake of ammonium and nitrate was measured as depletion from solutions by roots of intact 11 week old solution-cultured seedlings. Uptake rates of ammonium were consistently higher than those of nitrate in all experiments. Uptake rates for ammonium were 200% higher at pH 4 than at pH 6, but for nitrate were unchanged. Uptake rates of ammonium and nitrate were both reduced to a similar extent (70%) with a decrease in temperature from 20 °C to 10 °C. For ammonium uptake, there was rapid (<24 hr) adaptation to a reduction in root temperature. The apparent preference shown here for ammonium over nitrate could be indicative of E. nitens growing in cold, acidic forest soils where ammonium is commonly more available than nitrate. These results suggest that N uptake rates of E. nitens may be maximised under a wide variety of conditions if N is supplied predominantly in the ammonium form. This revised version was published online in June 2006 with corrections to the Cover Date.     

Local measurements of nitrate and potassium fluxes along roots of maritime pine. Effects of ectomycorrhizal symbiosis

In order to assess the actual role of ectomycorrhizae in ion uptake by the ectomycorrhizal root system, we used a microelectrode ion flux estimation methodology that provided access to local values of net fluxes. This made it possible to investigate the heterogeneity of ion fluxes along the different types of roots of Pinus pinaster associated or not with ectomycorrhizal species. We compared two fungi able to grow with nitrate in pure culture, Rhizopogon roseolus and Hebeloma cylindrosporum, the former having a positive effect on host tree shoot growth (c. +30%) and the latter a negative effect (c.? 30%). In non‐mycorrhizal plants (control), NO₃^– was taken up at higher rates by the short roots than by the long ones, whereas K⁺ uptake occurred mainly in growing apices of long roots. In mycorrhizal plants, H. cylindrosporum did not modify K⁺ uptake and even decreased NO₃^– uptake at the level of ectomycorrhizal short roots, whereas R. roseolus strongly increased K⁺ and NO₃^– fluxes at the level of ectomycorrhizal short roots without any modification of the fluxes measured along the fungus‐free long roots. The measurement of ion influxes at the surface of the ectomycorrhizal roots can provide a way to reveal actual effects of mycorrhizal association on ion transport in relation to mycorrhizal efficiency in natural conditions.     

冬小麦等4种作物对铵,硝态氮的吸收能力

采用水培试验探讨了冬小麦、大豆、油菜和莴笋４种作物对硝、铵态氮的相对吸收能力以及这两种氮源对它们生长发育的影响。试验表明：（１）不同氮源对供试作物的生长发育影响极大。供给硝态氮,这些作物生长发育良好,供给等量的ＮＯ＾－３和ＮＨ＾－４（１：１）时,蔬菜作物莴笋生长量下降幅度最大;供给铵态氨,莴笋和大豆极为敏感,供给ＮＯ＾－３时莴笋吸氮量显著高于供给等氮量ＮＯ＾－３和ＮＨ＾＋４,莴上麦供给等量ＮＯ＾－     

Localization and quantification of net fluxes of H+ along maize roots by combined use of pH-indicator dye videodensitometry and H+-selective microelectrodes

Two methods for measuring proton fluxes along intact maize roots grown with NH ₄ ⁺ or NO ₃ ⁻ at pH 6.5 were compared. Videodensitometric measurement of changes in a pH-indicator dye by video camera was used to map pH around roots and determine the amounts of protons released by various root regions. This method was compared with potentiometric determination of the concentration of H⁺ in the unstirred layer at the root surface using ion-selective microelectrodes. With NH ₄ ⁺ the roots released large amounts of H⁺ in preferential regions where the rate of flux can reach 1.4 or even 2.5 nmol m⁻¹ s⁻¹. Videodensitometry indicated a first region of root acidification in the subapical zone, but this was more difficult to localize with microelectrodes. With NO₃ ⁻ both methods showed that the roots released small amounts of H⁺ and that the apical region took up H⁺ in the first 10 mm then sometimes released H⁺ over the following 10 mm of root. The H⁺ flux profiles obtained by both methods were in good agreement in terms of both order of magnitude of the fluxes and spatial differences along the root. These results suggest that videodensitometry, which is easier to use than potentiometry, can be used to screen different plant species or cultivars under various experimental conditions. The microelectrode technique is indispensable, however, for studying the underlying mechanisms of net H⁺ fluxes. This revised version was published online in June 2006 with corrections to the Cover Date.     

The uptake of nitrate and ammonium nitrogen in Chara hispida L.: the contribution of the rhizoid

Abstract The uptake of ammonium and nitrate nitrogen by cultured plants of the green freshwater alga Chara hispida L. has been compared quantitatively with the contribution of its rhizoidal tissue. In the short-term, the rhizoid takes up 7–20% of the ammonium nitrogen, and about 15% of the nitrate that is taken up by whole plants under similar conditions. The uptake was studied over a range of both temperatures and external concentrations. The apparent activation energy for the uptake of NH₄⁺ and NO₃^? by the whole plant was found to be 50 kJ mol^?1 and 30 kJ mol^?1, respectively. For the rhizoid, the values were similar for both nitrogenous ions, 106 kJ mol^?1 and 70–100 kJ mol^?1. The rhizoidal uptake mechanism for ammonium nitrogen operates more efficiently compared to that in the whole plant. Nitrate is taken up by the rhizoid by a mechanism with a substrate affinity higher than in the plant taken as a whole. The possible ecological significance of the results is discussed.     

Replacement of nitrate by ammonium as the nitrogen source increases the salt sensitivity of pea plants. I. Ion concentrations in roots and leaves

Pea seedlings (Pisum sativum L. cv ‘Kleine Rheinlän-derin’) were grown hydroponically in solutions containing either nitrate (3 or 14 mol m⁻³) or ammonium (3 mol m⁻³) as the nitrogen source. Ammonium nutrition as such had no negative effect on plant biomass production, but drastically increased the sensitivity to moderate salinity (50 mol m⁻³ NaCl). The reasons for this effect are investigated here and in a subsequent paper. The appearance of visible symptoms of salt damage (wilting of marginal leaf areas followed by progressive necrosis) was paralleled by the development of several characteristic modifications in the solute and metabolite contents. Major changes were: (i) high salt (NaCl) accumulation in leaves; (ii) accumulation of ammonium (up to 20 mol m⁻³) and amino acids (up to 110 mol m⁻³) in leaves, but at decreased ammonium uptake rates; and (iii) decreased protein content. In a comparison paper we report on the subcellular distribution of salts, ammonium and metabolites under the above conditions.     

Protoplast ion fluxes: their measurement and variation with time, position and osmoticum

The ability to measure directly individual protoplast ion fluxes is a valuable addition to patch clamp and other techniques when using protoplasts to study membrane transporters. Before interpreting observations on protoplasts in terms of behaviour of intact cells and tissues, some methodological questions should be addressed. These include effects of space and time variations of transporter activities over the membrane, the osmotic dependence of specific ion transporters and the effect of the regenerating cell wall. In this study net H⁺ and Ca²⁺ fluxes were measured from individual corn (Zea mays L.) coleoptile protoplasts using a non-invasive microelectrode technique for ion flux measurements. For Ca²⁺, the flux distribution was almost symmetrical, ranging ±30 nmol · m⁻² · s⁻¹ around zero. For H⁺ it was skewed towards efflux ranging from −100 to +10 nmol · m⁻² · s⁻¹. The distribution of H⁺ fluxes through the protoplast surface was a complex mosaic which changed with time, sometimes showing oscillations. These flux variations with time and position around the surface, apparently driven by endogenous mechanisms, may be relevant to protoplast pH homeostasis. When the new cell wall was partially regenerated on the next day, the correlation between H⁺ and Ca²⁺ fluxes increased, which is consistent with the weak-acid Donnan-Manning model of cell wall ion exchange. Received: 11 June 1997 / Accepted: 10 July 1997     

Differences in nitrate and ammonium uptake between Scots pine and European larch

In forest soils, ammonium is usually the predominant form of inorganic nitrogen. However, the capacity of trees to utilize both NO₃ ^- and NH₃ ⁺ may provide greater flexibility in responding to changes of nitrogen supply from the environment. Such capacity has been studied in seedlings of Scots pine (Pinus sylvestris L.) and European larch (Larix decidua Mill.) grown in the presence or absence of either nitrate or ammonium. Nitrate-induced plants showed a higher nitrate uptake rate than non-induced plants; this difference was almost negligible after 24 h of exposure to NO₃ ^-. Ammonium uptake in both species was consistently higher than that of nitrate, regardless of prior nitrogen provision. In both nutrient conditions, larch showed a more efficient transport system in comparison with Scots pine, with higher ammonium and nitrate uptake rates in both induced and non-induced plants. This was consistent also with the activity of nitrate reductase, measured in vivo in roots and leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

The role of ammonium and nitrate retention in the acidification of lakes and forested catchments

The relative contribution of HN0₃ to precipitation acidity in eastern Canada has increased in recent years leading to some concern that the relative importance of NO^– ₃ deposition in acidification of terrestrial and aquatic ecosystems may increase. To gauge the extent of this impact, annual mass balances for N0^– ₃ and NH^{+ 4} were calculated for several forested catchments and lakes in Ontario. Retention of NH⁺ ₄ (R _NH4) by forested catchments was consistently high compared to retention of NO₃ ^– (R _NO3) which was highly variable. Retention of inorganic nitrogen was influenced by catchment grade and areal water discharge. In lakes, the reciprocals of retention of N0^– ₃ and NH⁺ ₄ were linearly related to the ratio of lake mean depth to water residence time (z/; equal to areal water discharge), and retention did not appear to be a function of degree of acidification of the lakes. Net N consumption-based acidification of lakes, defined as the ratio of annual NH; mass to N0^– ₃ mass consumption, was negatively correlated with / and N consumption-related acidification was most likely to occur when – was < 1.5 m yr^–1.If retention mechanisms are unaffected by changes in deposition, changes in deposition will still result in changes in surface water concentrations although the changes will be of similar proportions. Therefore, NO^– ₃ saturation should not be defined by concentrations alone, but should be defined as decreasing long-term, average NO^– ₃ retention in streams and lakes in response to long-term increases in NO^– ₃ deposition. Analysis o f survey data will be facilitated by grouping lakes and catchments according to similar characteristics.     

Ammonium and nitrate uptake by the floating plant Landoltia punctata

BACKGROUND AND AIMS: Plants from the family Lemnaceae are widely used in ecological engineering projects to purify wastewater and eutrophic water bodies. However, the biology of nutrient uptake mechanisms in plants of this family is still poorly understood. There is controversy over whether Lemnaceae roots are involved in nutrient uptake. No information is available on nitrogen (N) preferences and capacity of Landoltia punctata (dotted duckweed), one of the best prospective species in Lemnaceae for phytomelioration and biomass production. The aim of this study was to assess L. punctata plants for their ability to take up NH4+ and NO3- by both roots and fronds. METHODS: NO3- and NH4+ fluxes were estimated by a non-invasive ion-selective microelectrode technique. This technique allows direct measurements of ion fluxes across the root or frond surface of an intact plant. KEY RESULTS: Landoltia punctata plants took up NH4+ and NO3- by both fronds and roots. Spatial distribution of NH4+ and NO3- fluxes demonstrated that, although ion fluxes at the most distal parts of the root were uneven, the mature part of the root was involved in N uptake. Despite the absolute flux values for NH4+ and NO3- being lower in roots than at the frond surface, the overall capacity of roots to take up ions was similar to that of fronds because the surface area of roots was larger. L. punctata plants preferred to take up NH4+ over NO3- when both N sources were available. CONCLUSIONS: Landoltia punctata plants take up nitrogen by both roots and fronds. When both sources of N are available, plants prefer to take up NH4+, but will take up NO3- when it is the only N source.     

Net fluxes of ammonium and nitrate in association with H+ fluxes in fine roots of Populus popularis

Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH₄ ⁺) and/or nitrate (NO₃ ^?) to improve yields. However, little is known about net NH₄ ⁺/NO₃ ^? fluxes and their relation with H⁺ fluxes in poplar roots. In this study, net NH₄ ⁺/NO₃ ^? fluxes in association with H⁺ fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH₄ ⁺ and NO₃ ^? fluxes was found along root tips of P. popularis. The maximal net uptake of NH₄ ⁺ and NO₃ ^? occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH₄ ⁺ uptake was induced by ca. 48 % with provision of NO₃ ^? together, but net NO₃ ^? uptake was inhibited by ca. 39 % with the presence of NH₄ ⁺ in poplar roots. Furthermore, inactivation of plasma membrane (PM) H⁺-ATPases by orthovanadate markedly inhibited net NH₄ ⁺/NO₃ ^? uptake and even led to net NH₄ ⁺ release with NO₃ ^? co-provision. Linear correlations were observed between net NH₄ ⁺/NO₃ ^? and H⁺ fluxes in poplar roots except that no correlation was found between net NH₄ ⁺ and H⁺ fluxes in roots exposed to NH₄Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH₄ ⁺/NO₃ ^? uptake and that net NH₄ ⁺/NO₃ ^? fluxes and the interaction of net fluxes of both ions are tightly associated with H⁺ fluxes in poplar roots.     

Proton transfer along the external proton channel of bacteriorhodopsin

The process of proton transfer along a proton channel is considered using bacteriorhodopsin as a model system, for which a large body of experimental data is available. The possible amino acid composition of the external proton half-channel of bacteriorhodopsin and the stepwise scheme of proton transfer consistent with experimental data are proposed. The rate of proton transfer between fixed centers is assessed for certain regions of this channel for which spectroscopic data are available.     

Effect of root environment on proton efflux in wheat roots

Proton net efflux of wheat (Triticum aestivum L.) roots growing in sand culture or hydroponics was determined by measuring the pH values of the solution surrounding the roots by pH microelectrodes, by base titration and by color changes of a pH indicator in solid nutrient media. The proton net efflux was dependent on light, aeration, and source of nitrogen (NH ₄ ⁺ , NO ₃ ^? ). Ammonium ions caused the highest proton efflux, whereas nitrate ions decreased the proton efflux. Iron deficiency had no significant effect on proton efflux. Replacement of ammonium by nitrate inhibited proton efflux, whereas the reverse enhanced proton extrusion. A lag period between changes in plant environment and proton efflux was observed. The proton net efflux occurred at the basal portion of the roots but not in the root tips or at the elongation zone. Under optimal conditions, proton efflux capacity reached a maximum value of 5.7 μmole H⁺ g^?1 fresh weight h^?1 with an average (between different measurements) of 3.4 μmole H⁺ g^?1 fresh wth^?1 whereas the pH value decreased to 3.2–3.7 and reached a minimal value of 2.9. Inhibition of ATPase activity by orthovanadate inhibited proton efflux. The results indicate that proton efflux in wheat roots is ammonium ion and light dependent and probably governed by ATPase activity.     

不同基因型水生植物对铵态氮和硝态氮吸收动力学特性研究

针对不同营养状况的富营养化水体修复而选择吸收养分效率较强的水生植物,采用改进常规耗竭法比较研究了6种不同基因型水生植物凤眼莲(Eichhornia crassipes Solms)、黄花水龙(Jussiaea stipulacea Ohwi)、再力花(Thalia dealbata Fraser)、美人蕉(Canna glauca L.)、水芹[Oenanthe javanica(Bl).DC]和豆瓣菜(Nasturtium officinaleR.Br.)对铵态氮和硝态氮吸收动力学特性。结果表明,不同基因型水生植物吸收铵态氮和硝态氮的动力学特性可用Michaelis-Menten方程来描述。在低浓度培养下,不同基因型水生植物对NH4+-N和NO3--N吸收的动力学参数Imax和Km差异较大,其吸收NH4+-N和NO3--N的Imax最大是水芹,其次是豆瓣菜;Km值最小的是水芹,其次是豆瓣菜;且水芹对NH4+-N和NO3--N不仅具有较强的亲和力,还具有较高的离子吸收速率。结果还表明,当介质中氮浓度较低时,水芹有优先吸收硝态氮的趋势,而豆瓣菜和再力花有优先吸收铵态氮的趋势。